Principles of Ultrasound

Published on 06/02/2015 by admin

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Principles of Ultrasound

Christopher J. Gallagher and John C. Sciarra

Nature of Ultrasound: Compression and Rarefaction

Ultrasound is sound waves propagated through a medium at a frequency above that which we can hear. Imaging depends on displaying the time required for an ultrasound pulse to go to a cardiac structure and return. We acoustically challenged humans only hear from 20 cycles/second to 20 000 cycles/second, or 20 kilohertz (named after the famous physicist and car-rental magnate).

Ultrasound starts at 20 kilohertz (20 kHz). For our medical imaging, the frequency used is between 1 and 20 megahertz (1 and 20 MHz). Take a look at our probe, and you’ll see something like 5 or 7 MHz.

Keep in mind that sound, or ultrasound, must get propagated through a medium.

Jimi Hendrix blasted his guitar through the AIR at Woodstock. You blast your ultrasound through the TISSUE and FLUID with your TEE. Remember the ads when Alien came out? “In space, no one can hear you scream.” That’s right, there’s nothing to propagate in a vacuum. There’s no air for you to compress and rarefy.

Ultrasound does not propagate in air; this will be a recurring problem. Ultrasound only propagates through tissue and fluids.

Frequency, Wavelength, and Tissue Propagation Velocity

Note that frequency is the number of complete cycles per second, and wavelength is the distance from one corresponding area to the next (usually peak to peak). Propagation velocity is the wavelength ∞ frequency.

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How does that relate to us? The propagation velocity of sound waves in human tissues is 1540 meters/second. So, since the velocity is pretty constant, that means the time it takes to go “out and back” correlates with distance. Time vs distance is the basis of all “bounce technology” (sonar in a ship, locating enemy submarines; Doppler radar letting us know about a coming rainstorm; TEE telling us where the aortic dissection started).

Wavelength is important because resolution (the ability to tell two things apart) is no better than 1 or 2 wavelengths. So if you have a long, long wavelength, you won’t be able to tell things apart very well. If you have a short, short wavelength, you will be able to tell tiny things apart. This comes into play when you are adjusting the wavelength for near and far objects.

A frequent consideration that occurs frequently with frequency is this: the higher the frequency, the better the resolution but the shallower the penetration.

The flip side, or the wavelength paradigm, is also true: the longer the wavelength, the deeper the penetration but the worse the resolution. The take-home message for budding TEE’ogists? To see an object close up, go to a higher frequency, and you’ll see it in more detail. For a distant object—say, the pulmonic valve, which lies far from the TEE probe—use a longer wavelength (or, in other words, a lower frequency). This makes sense if you remember that frequency and wavelength are inversely related:

Ultrasound–Tissue Interactions

Here’s a little commonsense tip for ultrasound and tissue. Ultrasound in its usual diagnostic form doesn’t hurt tissue. A zillion kids have been bombarded with ultrasound waves in utero, and, except for a fondness for Call of Duty: Black Ops, bare midriffs, and pierced cartilage, there seems to have been no lasting damage. (Of course, cranked to the max, sound can crack stones, as we see in the ESWL suite every day.) But keep in mind that the probe is a machine that converts some of its energy to heat. So don’t leave the probe running forever, lest you cause a burn to the esophagus. Turn the TEE off after you’ve done your study and let it cool.

Reflection

Ultrasound is based on reflection of the signal from internal structures. Ultrasound is reflected at tissue boundaries, and that is what allows us to see where, for example, the ventricle ends and the blood begins. The ultrasound beam goes through tissue of one impedance, hits tissue of a second impedance, then reflects back to the transducer. Impedance depends on tissue density and on propagation velocity through the tissue. For our purposes, tissue density is the most important. Heart muscle has higher impedance than blood (it’s thicker, after all).

“Blood is thicker than water,” most people know by rote. Few know that the second half of that folk saying is: “…but the tissue impedance of ventricular, atrial, and valve structures is higher than blood tissue impedance.”

Since reflection is the key to the kingdom, and you prefer a “straight on” bounce coming back to your transducer, it makes sense that your best view is straight on, at 180 degrees to the transducer. At any angle other than 90 degrees, some of the signal will bounce “away” from the transducer.

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